1. Field of the Invention
The present invention relates to an engine exhaust emission control system including a ternary catalyst, and more particularly, to an engine exhaust emission control system designed so that hydrocarbons in an exhaust gas are adsorbed by a hydrocarbon adsorber, when the ternary catalyst is inactive, such as during a cold start.
2. Description of the Prior Art
A prior art engine exhaust emission control system is described in Japanese Patent Application Laid-Open No. 279719/92.
This prior art engine exhaust emission control system includes a hydrocarbon adsorber or adsorbing device provided at an upstream location in an exhaust passageway extending from an engine, while an oxidizing catalyst is provided at a downstream location, and a heat exchanger is provided between a location upstream of the hydrocarbon adsorber in the exhaust passageway and a location upstream of the oxidizing catalyst in the exhaust passageway. In this system, hydrocarbons in the exhaust gas are adsorbed by the hydrocarbon adsorber and prevented from being released to the atmosphere, when the exhaust gas is at a lower temperature. The hydrocarbons adsorbed by the hydrocarbon adsorber are desorbed from the hydrocarbon adsorber into the exhaust gas when the temperature of the exhaust gas increases. The hydrocarbons are passed through the heat exchanger where the temperature thereof increases to a level equal to or greater than a catalyst activating temperature. The hydrocarbons are thus processed in the oxidizing catalyst located downstream of the heat exchanger, and are therefore prevented from being released to the atmosphere.
However, the above prior art system has a problem because no oxidizing catalyst exists upstream of the hydrocarbon adsorber and it is therefore necessary to adsorb all of the hydrocarbons discharged from the engine by the hydrocarbon adsorber until the temperature of the oxidizing catalyst increases to a level equal to or greater than the activating temperature. This results in an increase in size of the hydrocarbon adsorber. The above prior art system includes a heat exchanger for raising the temperature of the exhaust gas, but the heat exchanger and the oxidizing catalyst are spaced apart from each other. This results in poor catalyst temperature raising effect and in a disadvantage with respect to space.